Ion concentration polarization (ICP) is a fundamental electrochemical phenomenon that describes the mismatch of charge carriers at the nanoporous membrane or nanochannel interface (Rubinstein, et al., Journal of Chemical Society Faraday Transactions II, 75, 231 (1979); Holtzel, et al., Journal of Separation Science 30, 1398 (2007)). Significant concentration gradient can be generated when ion current is transported through perm-selective membranes (nanochannels) or electrodes, altering the overall conduction properties of these electrochemical systems. Understanding ICP and related transport phenomena is important in many engineering fields such as bio-sensing, and fuel cells application. (Wang, et al., Anal. Chem., 77, 4293 (2005); Wang, et al., Lab Chip, 8,392 (2008); Song, et al., Power Sources, 183, 674 (2008)). Classical theory of ICP, originally developed by Nemst in 1904, predicts that there will be a maximum current allowed through the perm-selective membrane (limiting current behavior), because ion concentration in the anodic side of the membrane reaches near zero. At this point (called as limiting current condition) no further increase in ion current through membrane is possible even if the bias is further increased. (Probstein, et al., Physicochemical Hydrodynamics: An Introduction (Wiley-Interscience, 1994)). However, in reality, significant over-limiting current can always be observed experimentally in most perm-selective membranes. The detailed physical mechanism of over-limiting current has been in debate since as early as 1979. (Rubinstein, et al., supra). Overlimiting current is often associated with water dissociation at the vicinity of the membrane, but recently, Kim, et al. experimentally confirmed the existence of strong convection of fluid layer near the membrane, in direct coincidence with the formation of depletion zone (diffusion layer) as well as the classic over-limiting current behavior. (Strathmann, et al., J. Membr. Sci., 125, 123 (1997); Kim, et al., Phys. Rev. Lett., 99, 044501 (2007)). The importance of fluid flow in ICP and limiting current behavior is now well established, both theoretically and experimentally. (Rubinstein, et al., Phys. Rev. E, 62, 2238 (2000); Pundik, et al., Phys. Rev. E, 72 (2005); Rubinstein, et al., Phys. Rev. Lett., 101, 236101 (2008)).
Electrochemical systems using microfluidic technology are becoming important for a variety of fields including fuel cells and biosensors. (U.S. Patent Publications 20090242406, 20080253929, 20060292407 and U.S. Pat. Nos. 6,444,339 and 7,381,858).
In most electrochemical membrane applications, limiting current behavior is the source of concentration over-potential, which significantly limits the ion/chemical transport through the membrane. As such, a need exists to develop a method for effectively reducing or preventing limiting current behavior. A reduction in limiting current behavior would potentially enhance the electrochemical membrane performance significantly.